(1) Field of the Invention
The present invention relates to a method and an apparatus for processing exhaust gas produced during manufacturing semiconductor devices. Particularly, it relates to a method and an apparatus for processing an exhaust gas produced during manufacturing semiconductor devices, in which a thermally disintegrable or thermally oxidizable gas is cost-effectively disintegratable or oxidized into harmless substances so that resulting fine particles and a processed gas can easily be discarded.
(2) Description of the Related Art
These days, the progress of computer technology and electronic control apparatuses making use of this technology is rapidly being made. It seems that no one can predict in which direction the progress is going to be made and how far it goes. Consequently, a progress in manufacturing technology for semiconductor devices, which play an important role as electronic devices within a computer, is also rapidly being made, and its production is rapidly increasing.
Usually, semiconductor materials used for making these semiconductor devices are germanium (Ge) and silicon (Si). Materials such as gallium arsenide (GaAs) and Gallium phosphate are also used in commercial production of devices designed to have special functions.
Production of semiconductor devices usually includes the steps of forming cylindrical body made of semiconductor material, slicing the cylindrical body into semiconductor wafers, forming a number of devices by repeatedly masking, filming, doping, etching, etc., and cutting the wafer into individual devices.
During such production steps, exhaust gases which contain toxic and harmful substances are also produced. In order to prevent environmental pollution, dumping of such exhaust gases without processing is strictly prohibited.
Examples of those materials which are used, or produced, during manufacturing semiconductor devices include those which contain silicon, arsenic, phosphorus, boron, metallic hydrogen, or flon. Halogen, halogenide, nitrogen oxide and others are also among the examples.
Examples of harmful gases which contain silicon are monosilane (SiH.sub.4), dichlorosilane (SiH.sub.2 Cl.sub.2), silicon trichloride (SiHCl.sub.3), silicon tetrachloride (SiCl.sub.4), silicon tetrafluoride (SiF.sub.4), disilane (SiH.sub.6), TEOS, etc.
Examples of harmful gases which contain arsenic are arsine (AsH.sub.3), arsenic fluoride (III) (AsF.sub.3), arsenic fluoride (V) (AsF.sub.5), arsenic chloride (III) (AsCl.sub.3), arsenic chloride (V) (AsCl.sub.5), etc. Examples of harmful gases which contain phosphorus are phosphine (PH.sub.3), phosphorus fluoride (III) (PF.sub.3), phosphorus fluoride (V) (PF.sub.5), phosphorus chloride (III) (PCl.sub.3), phosphorus chloride (V) (PCl.sub.5), phosphorus oxychloride (POCl.sub.3), etc.
Examples of harmful gases which contain boron are diborane (B.sub.2 H.sub.6), boron trifluoride (BF.sub.3), boron trichloride (BCl.sub.3), boron tribromide (BBr.sub.3), etc., and examples of those which contain metallic hydrogen are hydrogen selenide (H.sub.2 Se), monogermanium (GeH.sub.4), hydrogen telluride (H.sub.2 Te), stibine (SbH.sub.3), tin hydride (SnH.sub.4), etc. Examples of harmful gases which contain flon are methane tetrafluoride (CF.sub.4), methane trifluoride (CHF.sub.3), methane difluoride (CH.sub.2 F.sub.2), methane hexafluoride, propane hexafluoride (C.sub.3 H.sub.2 F.sub.6), propane octofluoride (C.sub.3 F.sub.8), etc.
Examples of halogens and halogenides, which are harmful in a gaseous state, usually include fluorine (F.sub.2), hydrogen fluoride (HF), chlorine (Cl.sub.2), hydrogen chloride (HCl), carbon tetrachloride (CCl.sub.4), hydrogen bromide (HBr), nitrogen trifluoride (NF.sub.3), sulfur tetrafluoride (SF.sub.4), sulfur hexafluoride (SF), tungsten fluoride (VI) (WF.sub.6), molybdenum fluoride (VI) (MoF.sub.6), germanium tetrachloride (GeCl.sub.4), tin tetrachloride (SnCl.sub.4), antimony chloride (V) (SbCl.sub.5), tungsten chloride (VI) (WCl.sub.6), molybdenum hexachloride (MoCl.sub.6), etc.
Examples of nitrogen oxides, which are also harmful in a gaseous state, include nitric oxide (NO), nitrogen dioxide (NO.sub.2), nitrous oxide (N.sub.2 O), etc. Examples of other harmful gases include hydrogen sulfide (H.sub.2 S), ammonia (NH.sub.3), trimethylamine ((CH.sub.3).sub.3 N), etc.
Moreover, it is also known that fine particles are formed in a gas containing ethane (C.sub.2 H.sub.6) or propane (C.sub.3 H.sub.8) which are flammable, nitrogen (N.sub.2), oxygen (O.sub.2), argon (Ar), carbon dioxide (CO.sub.2), etc.
These days, public awareness toward prevention of environmental pollution is widely recognized. Therefore, a discharge of exhaust (waste) gases which contain such harmful substances and fine particles are strictly prohibited, and it is required that such waste gases be processed and treated so that they are discharged to an outside environment in a safe and clean condition.
In order to meet such requirements, a so-called exhaust gas processing apparatus which consists of an exhaust duct and a harmful substance processing unit is conventionally employed, where the exhaust duct guides an exhaust gas from the production line of semiconductor devices to the processing unit and the processing unit disintegrates the exhaust gas by a catalyst or adsorbs harmful substances or fine particles thereof by an adsorbent so as to make the exhaust gas harmless. This exhaust gas processing apparatus either chemically processes or physically removes the harmful substances so that the processed gas can be released into the atmosphere.
However, such conventional methods and apparatuses involve complicated processing of the exhaust gas. One method uses an expensive catalyst to disintegrate the gas, and another method uses an expensive adsorbent to treat the gas. Therefore, these catalyst or adsorbent contain or hold harmful substances in one form or another and, therefore, cannot be discarded per se but must be disposed of as industrial wastes. This raises the processing cost.
Furthermore, since the expensive catalysts cannot be used repeatedly and they must be disposed of as industrial wastes, the processing cost further rises.
In order to avoid the above-mentioned disadvantages, it is being considered these days that thermally disintegrable or thermally oxidizable exhaust gases produced during manufacturing of semiconductor devices can be processed by heat so that the exhaust gas becomes harmless. For example, if monosilane (SiH4) is heated in air, it changes to silicon dioxide and water, which are harmless.
Furthermore, if such thermally disintegratable or thermally oxidizable exhaust gases are heat-processed, they change to solids which are harmless and easy-to-handle or to acid gases (NO.sub.x such as NO, N.sub.2 O.sub.3, or SO.sub.x such as SO.sub.2, SO.sub.3) which are also easy to handle in further processing because their adsorptivity to water or alkaline water is enhanced so that they can easily be removed.
However, fine particles of solid oxides produced by heat-processing of an exhaust gas and/or fine particles formed during manufacturing of semiconductor devices and already present in the exhaust gas are extremely small in the mean diameter. Therefore, they can barely be removed by methods such as venturi scrubber, scrubber dust collector (wet dust scrubber), cyclone scrubber.
A bag filter and/or an electrostatic precipitator (ESP) are the most effective means for removing such fine particles. However, maintenance of the electrostatic precipitator becomes considerably difficult to perform when the amount of the fine particles are large. Therefore, the electrostatic precipitator is not practical.
Moreover, the bag filter is easily dogged with fine particles, which causes a sudden decrease in pressure and variations in both pressure and air flow. This variation is considerably difficult to control, and prolonged effective dust removal cannot be expected.